The present invention relates to the field of neurosurgery, and provides a device which facilitates the implantation of bone into the spine following removal of vertebrae, and which also facilitates the fusion of the implanted bone with the surrounding bone. The invention also includes a method of performing spinal surgery, and in particular, of stabilizing the spine following removal of one or more vertebrae.
Cancer or trauma or degenerative changes can cause parts of the human vertebrae to develop outgrowths or ridges that can touch the spinal cord and cause pain and/or paralysis. Neurosurgeons have developed means of treating such conditions, by removing part of the vertebrae, and, where appropriate, replacing the removed bone with something else. The removal of all or part of a vertebra is called a "corpectomy" or a "vertebrectomy". In some cases, one can replace the bone removed by corpectomy with bone taken from another site on the body of the patient; in other cases, one can obtain bone from a "bone bank". Given the right conditions, the new bone material will fuse to the bone surrounding the corpectomy site, and can become for practical purposes a part of the patient's body. To achieve the desired fusion, one must stabilize the spine so that the bone has time to fuse. The fusion process can take from six weeks to six months.
In performing spinal surgery, one can approach the spine either from the front (anterior) or rear (posterior) sides. The posterior approach has the disadvantage that since the vertebrae lie on the anterior side of the spinal cord, the surgeon must navigate past the spinal cord before reaching the vertebrae, and must take special care not to disturb the spinal cord. Conversely, with the anterior approach, the surgeon does not encounter the spinal cord while en route to the vertebrae. The present invention concerns the anterior approach.
The prior art contains many systems for stabilizing various parts of the spine following surgery. The development of such systems has made it possible to treat certain lesions of the spine aggressively, instead of simply immobilizing them in a brace. The typical external immobilizing device of the prior art comprises the halo vest. The typical internal immobilizing device comprises the Caspar plate, described below.
The Caspar plate system, named after Dr. Wolfhard Caspar, comprises a means for stabilizing the spine after anterior spinal surgery. The Caspar system includes a set of plates which one attaches to the remaining vertebrae surrounding the corpectomy site. In the Caspar procedure, one screws a plate directly onto the spine, the screws approaching within about one or two millimeters of the spinal cord. The Caspar system provides immediate stabilization of the spine following a corpectomy, and in other cases where the spine has become unstable following an accident. The Caspar system also eliminates the need for wearing the very cumbersome halo vest, and eliminates the need to undergo a separate surgical procedure from the rear.
However, the Caspar system also has disadvantages. It requires a large inventory of expensive equipment, including screws and plates of all sizes. The latter expense can represent a formidable obstacle to many medical institutions. Also, one needs to insert the screws through the spine, engaging the posterior cortex. Although one can monitor the position of the screws with an appropriate real-time viewing apparatus, the procedure carries the potential risk of spinal cord injury or laceration of the vertebral artery. When a competent surgeon performs the procedure, these complications rarely occur, but other complications such as loosening of the screws and persistent instability may develop. Moreover, the difficulty of the procedure discourages many surgeons from even attempting the anterior plating procedure.
The Synthes cervical spine locking plate constitutes another anterior plating system of the prior art. In the Synthes system, one inserts a second screw into the head of the anchor screw, thus creating a second affixation of the plate to the vertebrae. Many regard the Synthes system as easier, safer, and faster to use than the Caspar plate system, because the anchor screw does not penetrate the posterior cortex and because one therefore does not need to monitor the precise position of the screw during insertion. However, the Synthes locking plate has less versatility than the Caspar plate, as it provides the ability to fuse only two to three levels of the cervical spine.
Both the Caspar and Synthes systems also have the disadvantage that they do not work well in patients with osteoporosis, rheumatoid arthritis, ankylosing spondylitis, and other conditions of poor bone growth or metabolic bone disease.
Both the Caspar and Synthes systems have additional disadvantages inherent with the use of screws. First, as mentioned above, screws do become loose. If one uses the screws as the primary means of affixing the stabilizing device to the spine of the patient, loosening of the screws represents a major problem. Moreover, the use of screws presents a technical challenge to the surgeon. Correct screw placement requires experience, as well as a large inventory of expensive equipment, as well as imaging devices for monitoring the position of such screws. Also, with screw-based systems of the prior art, the surgeon must create a large opening in the patient, so as to view the screw along its shaft. Such an opening creates additional risks to the patient, such as the risk of injury to vascular structure and to nearby nerves.
In addition to the problem of how to stabilize the spine immediately after performing a corpectomy, vertebral surgery poses problems relating to the replacement of the removed bone. Some systems of the prior art require the use of a bone strut to replace the diseased bone segments removed in surgery. This bone grafting material costs a great deal, and sometimes one cannot obtain enough material when performing multiple vertebrectomies. Furthermore, bone graft material, usually taken from cadavers, has typically been sterilized by radiation, a process believed to weaken or destroy the strength and osteoconductive properties of bone. While it is possible to use other means of sterilization, such as ethylene oxide or freeze drying, it usually turns out that the best bone graft material comes from the patient, because the patient's own bone will likely fuse more rapidly than bone obtained elsewhere. Unfortunately, harvesting such bone consumes substantial time, involves substantial pain to the patient, and presents other risks, such as risk of infection at the harvest site, hemorrhage, and peripheral nerve injury.
The present invention overcomes the disadvantages of the prior art systems described above. First, the invention provides a device which surgeons can learn to use very easily, and which they can insert without intraoperative fluoroscopy or other means of accurately monitoring the position of a device within the body. Most neurosurgeons can use the device of the present invention with instruments already in their possession.
Secondly, the invention provides an adjustable device which can fit a large range of patients. This feature eliminates the need to keep a large inventory of parts in order to accommodate every possible patient.
Thirdly, the device allows one to use the patient's own cancellous bone which one removes during the vertebrectomy, possibly with the addition of further cancellous bone material from an external source. In any event, the invention reduces or eliminates the need to obtain a pelvic bone autograft from the patient.
The device of the present invention also reduces or eliminates the problem of loosening of screws, which can occur with the plating systems of the prior art, and which clearly can cause substantial pain and expense.